Casehardening salt bath



phere. molten bath with carbonaceous material, such as Patented Sept. 2, 1941 CASEHARDENING SALT BATH Paul H. Kramer, Detroit, Micln, assignor to Park Chemical Company, Detroit, Mich, a corporation of Michigan No Drawing. Application June 15, 1940, Serial No. 340,769

'7 Claims. (Cl. 148-15) The present invention relates to the case hardening of iron, steel or alloy steel articles in a molten salt bath.

It is the general object of the invention to provide an improved case hardening method and molten salt bath of the cyanide type in which the cyanide decomposition, evaporation, and heat loss are reduced to a minimum.

Another object is to provide an improved case hardening bath which is easy to remove from the casehardened articles even after an oil quench.

Another object of the invention is to provide an improved cyanide bath which may be used with a carbonaceous cover in order to prevent decomposition, evaporation, and heat loss.

Another object of the invention is to provide an improved heat treating bath of the type used for producing thin carburized cases, which bath is characterized by its unusually low cyanide con- Other objects and'advantages of the invention will become apparent from the followin specification and the appended claims.

The present invention is based on the discovery that certain phosphorus compounds, when added to a cyanide bath, will react with the cyanide salt to facilitate the formation of a carburized and nitrogen case andtwill do so without the necessity of exposing the molten bath to atmos- As a result, it is possible to cover the .oxygen compound of phosphorus to any conventional fused cyanide salt bath.

Conventional cyanide ,baths are made up of from 25 to 100 per cent of a commercial alkali cyanide, usually in the form of sodium cyanide.

As a general rule, the cyanide is combined with two or more inert salts which are present to form the bulk of thebath and to lower its melting point. The inert portion of the bath may comprise a eutectic mixture of sodium chloride and potassium chloride, or it may comprise a eutectic mixture of sodium chloride and sodium carbonate, in accordance with the usual practice. The nature of the inert salts used is not important so "far as the present invention is concerned, except that they must be alkali salts of the type commonly used in cyanide baths.

' Reactions #3 and #4.

In accordance with the present invention, there is added to a fused cyanide salt bath of the above class from .5 to 10 per cent of an alkali metal salt of an oxygen compound of phosphorus. Examples of such salts which are quite satisfactory include:

Tetra sodium pyrophosphate (Na4Pz01) Tri sodium phosphate (NaaPO4) Sodium meta phosphate (NaPOa) Sodium phosphite (NazHPO3) Disodium phosphate (NazHPOQ Mono sodium phosphate (NaHzPO4) and also potassium salts of the above. The preferred-phosphorus compound is tretra sodium pyrophosphate (Na4P2o'l), principally because it may be secured at low cost in an anhydrous form. Tri sodium phosphate (NaaPOr) is equally satisfactory, except for the fact that it cannot be obtained in an anhydrous form, with the result that it releases water in the form of steam when first added to the molten bath. As a matter of fact, tetra sodium pyrophosphate breaks down into tri sodium'phosphate in the bath, and it is the tri sodium phosphate which reacts with the sodium cyanide. The reactions which are believed to occur are substantially as follows:

It will be noted that the tri sodium phosphate which enters into Reaction #2 reappears in Reaction #5 without any loss, hence the Phosphorus compound is not used up in the case hardening reaction. However, an equilibrium is apparently reached eventually because fresh phosphorus compound must be added after a period of use to activate the bath. The sodium cyanide which enters into Reaction #2 is partially reformed in Reaction #3. The remainder of the sodium reappears in sodium carbonate formed in Since, in the above described bath, the oxygen required to decompose the cyanidesalt is supplied by the phosphorus compound, it is unnecessary to have the surface of the bath exposed to the atmosphere, with the result that it is possible to cover the surface of the bath with powdered graphite. This practice is preferred, since the graphite layer, or' cover, excludes atmosphere to a large extent and thereby reduces decomposition of the cyanide, lessens the evapoaccordance with the present invention is as follows:

Per cent Na4P201 3 NaCN 40 Powdered graphite .75 NazCOa 25 and NaCl Remainder The above formula is the formula for the makeup mixture. In actual use, a bath to which the above formula is added will balance at a sodium cyanide content of from to per cent. This particular bath is especially suitable for light cases (up to .015 inch) and gives uniformly good results when the sodium cyanide content is maintained at approximately 15 to 20 per" cent, or higher.

Any desired quantity of cyanide may be used in the bath. In general, the depth of case formed during a given period increases with the percentage of cyanide up to 15 or 20 per cent. However, it is. found that with increased quantities of the phosphorus compound less cyanide is necessary. Thus, a bath containing 10 per cent Na4P20'z will operate satisfactorily with 8 per cent cyanide, or even less. The phosphorus compound, if increased beyond 10 per cent, may cause objectionable corrosion of the pot.

It is found that the improved case hardening bath of the present invention is of particular utility where it is desired to form a very thin case, say, from .001 to .003 inch in depth, since for such use as little as 5 per cent sodium cyanide is required provided one of the specified phosphorus compounds is present. This contrasts with other sodium cyanide baths for the same purpose which require at least 15 per cent, and preferably per cent sodium cyanide.

The performance of the bath of the present invention may be indicated by the following examples:

A bath containing 2 per cent Na4P2O7 and 11 per cent NaCN at a temperature of 1650 F. produced a total case of .017 inch in one hour. .028 inch in two hours, and .125 inch in sixteen hours.

A bath containing 3 per cent Na4P2O1 and 19 per cent NaCN at 1650 F. produced a total case of .019 inch in one hour, .030 inch in two hours, and .040 inch in three hours.

A bath containing 3 per cent Na4PzO1 and 23 per cent NaCN at 1650 F. produced a total case of .020 inch in one hour, .030 inch in two hours, and .045 inch in four hours. This same bath for a period of several days produced a case of .035 inch in three hours. Approximately the same results were secured with NaaP04 and NazHPOs.

A bath containing 10 per cent Na4P2O-1 and 10 per cent NaCN at 1650 F. produced a total case of .038 inch in three hours. After this bath had been used for 40 hours, so that the NaCN content had dropped to 2.5 per cent, it produced a total case of .026 inch in three hours. No additions of any kind were made to the bath during the 40 hour period.

The method and bath of the present invention are quite economical because of the reduced amounts of cyanide required, as well as the reduction in the rate of cyanide decomposition, the rate of evaporation and the loss of heat. Moreover, they are easy to clean off the case hardened parts after they have been given an oil quench. The baths of the present invention produce high quality, uniform cases in a short period of time, and the cases contain both carbon and nitrogen. In addition, it is noted that when no carbonate is present the bath s of the present invention produce a case having a very high nitrogen content.

While a number of specific examples have been given, it is apparent that variations in the ingredients and percentages of ingredients may be had without departing from the spirit of the invention or the scope of the appended claims.

What is claimed is:

1. The method of case hardening ferrous metal parts which comprises immersing the parts in a molten salt bath containing an alkali metal cyanide while protecting said bath from atmosphere by a covering layer of graphite and decomposing the cyanide by reaction with an alkali metal salt of an oxygen compound of phosphorus, the remainder of said bath being essentially made up of alkali metal salts.

2. The method of case hardening ferrous metal parts which comprises immersing the parts in a molten bath containing an alkali metal cyanide and an alkali metal salt of an oxygen compound of phosphorus, the remainder of said bath being essentially made up of alkali metal salts, and maintaining a carbonaceous cover upon the top of the bath during the case hardening operation.

3. A case hardening salt bath containing as active ingredients an alkali metal cyanide and an alkali metal salt of an oxygen compound of phosphorus, and covered by a layer of carbonaceous material, the remainder of said bath being essentially made up of alkali metal salts.

4. A case hardening salt bath composition containing as active ingredients from 2.5 to 99.5 per cent alkali metal cyanide and from 10 to .5 per cent tetra sodium pyrophosphate, the remainder of said bath being essentially made up of alkali metal salts.

5. A case hardening salt bath composition containing as active ingredients from 2.5 to 99.5 per cent alkali metal cyanide and from 10 to .5 per cent tri sodium phosphate, the remainder of said bath being essentially made up of alkali metal salts.

6. A case hardening bath containing as active ingredients from 2.5 to 99.5 per cent alkali metal cyanide and from 10 to .5 per cent tetra sodium pyrophosphate, and approximately .75 per cent powdered graphite, the remainder of said bath being essentially made up of alkali metal salts.

7. A case hardening bath containing as active ingredients from 2.5 to 99.5 per cent alkali metal cyanide and from 10 to .5 per cent tri sodium phosphate, and approximately .75 per cent powdered graphite, the remainder of said bath being essentially made up of alkali metal salts.

PAUL H. KRAMER. 

